1. Field of the Invention
The present invention relates to a method for manufacturing organic thin films having a uniform structure and free from defects, based on the Langmuir-Blodgett technique.
2. Description of the Related Art
In recent years, electrical elements utilizing an organic thin film manufactured by the Langmuir-Blodgett technique (to be referred as the LB technique hereinafter) have been developed and studied. In order to apply an organic thin film manufactured by the LB technique to an electrical element, it is important to manufacture a built-up film having a uniform structure and free from defects.
A conventional film manufacturing apparatus for an organic thin film by the LB technique basically comprises a trough for forming a liquid surface on which a monomolecular film is to be developed, a barrier for compressing the developed monomolecular film, a drive device for the barrier, and a surface pressure balance for detecting a surface pressure of the monomolecular film. The apparatus includes two types of apparatus, i.e., an apparatus which has one barrier and moves the barrier in one direction, and an apparatus which has two barriers, and moves the barriers in two directions. When organic thin films are manufactured based on the LB technique using the above-mentioned apparatus, organic molecules are dissolved in a developing solution such as chloroform, and the solution is dripped and developed drop by drop on the liquid surface of the trough by an injector. The molecules developed on the liquid surface are compressed by driving the barrier. In this case, the surface pressure of the monomolecular film is monitored by the surface pressure balance as a surface molecular density of the monomolecular film, and the monomolecular film is compressed to obtain predetermined surface pressure .pi.. Then, a substrate is moved perpendicularly to the liquid surface on which the monomolecular film is developed, so that the developed monomolecular film is transferred and built up onto the substrate.
Since the surface pressure balance for detecting the compressed state of the monomolecular film on the liquid surface, the substrate on which the monomolecular films are built up, and the barrier for compressing the monomolecular film are displaced from each other, problems occur. More specifically, when the monomolecular films are built up on the substrate, the monomolecular film on the liquid surface is decreased near the substrate, and the surface pressure is also decreased. The conventional film manufacturing apparatus is operated under the assumption that a decrease in surface pressure upon building-up of the monomolecular film is immediately transmitted to the entire monomolecular film is averaged, and is detected by the surface pressure balance. Thus, the barrier is moved in a compression direction until the monomolecular film recovers the predetermined surface pressure. With this operation, the building-up operation of the monomolecular film is assumed to be performed constantly at the predetermined surface pressure.
However, molecules which can exhibit properties according to the above assumption are only limited ones such as aliphatic molecules. In contrast to this, since monomolecular films formed of molecules such as dye-containing molecules and macromolecules exhibit large viscosity, a decrease in surface pressure of the monomolecular film near the substrate is not always immediately transmitted to the surface pressure balance separate from the substrate. The viscosity of the monomolecular film developed on the liquid surface means propagation, relaxation characteristics of a stress (surface pressure) generated when a given surface strain acts on the monomolecular film. More specifically, the viscosity of the monomolecular film delays propagation time of a change in surface pressure to the surface pressure balance. Therefore, when the building-up operation of dye-containing molecules or macromolecules is continued based on the surface pressure detected by the surface pressure balance, the surface density of the monomolecular film near the substrate is continuously decreased. As a result, the density of the monomolecular film built up on the substrate is not constant, and a built-up film having a uniform structure free from defects cannot be obtained. In order to transfer the monomolecular film on the liquid surface onto the substrate, the surface pressure of the monomolecular film must be equal to or higher than a given value. This value varies depending on types of molecules, hydrophilic and hydrophobic properties and moving directions of the substrate. Therefore, under a given condition, the monomolecular film may be able to be successfully built up but may not be built up if the condition changes.
Furthermore, when the barrier, substrate, and surface balance are arranged separate from each other, molecules radially flow from a portion around the substrate toward the substrate. For this reason, even if building-up of the monomolecular film is possible, nonuniformity of molecules occur along the boundary between the substrate and the monomolecular film, and a uniform monomolecular film cannot be obtained.
In order to prevent nonuniformity of the molecular density distribution in the conventional apparatus, the building-up rate of the monomolecular film may be sufficiently decreased so as to eliminate the influence of the viscosity of the monomolecular film, i.e., so as to accurately transmit the surface pressure of the monomolecular film near the substrate to the surface pressure balance. However, with this method, it takes a long period of time i.e. 10 to 1,000 times the existing method, and this method cannot be put into practical applications.